Replaceable electrode surfaces for high field electrostatic lenses

ABSTRACT

A two-piece electrode assembly consists of a permanent mounting fixture and a replaceable, polished electrode surface that snaps on the mounting fixture. When this electrode surface in an electrostatic lens becomes pitted, it can easily be snapped off and a new surface snapped on without need for realignment.

United Stts atent 1 Seliger et a1.

[451 Set. 24, 1974 REPLACEABLE ELECTRODE SURFACES FOR HHGH HELD ELECTROSTATIC LENSES [75] Inventors: Robert L. Seliger, Agoura; Wayne P.

Fleming, Newbury Park, both of Calif.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: Aug. 24, 1973 [21] Appl. No.: 391,414

[52] US. Cl 313/237, 313/49, 313/82 HF [51] llnt. Cl. Htllj 1/00 [58] Field of Search 313/236, 237, 49, 82 BF, 313/82 R [56] References Cited UNITED STATES PATENTS 3,042,828 7/1962 Josephson 313/237 X 3,114,068 12/1963 Shortride 313/237 3,328,618 6/1967 Wilson 313/82 TS 3,334,262 8/1967 Nelson 313/237 3,462,629 8/1969 Bell 313/82 TS X Primary ExaminerJames W. Lawrence Assistant Examiner-Saxfield Chatmon, Jr.

Attorney, Agent, or FirmR. S. Sciascia; P. Schneider; W. Ellis [5 7 ABSTCT A two-piece electrode assembly consists of a permanent mounting fixture and a replaceable, polished electrode surface that snaps on the mounting fixture. When this electrode surface in an electrostatic lens becomes pitted, it can easily be snapped off and a new surface snapped on without need for realignment.

8 Claims, 7 Drawing Figures HIGH-VOLTAGE ELECTRODE HIGH FIELD REGION PATEN'I'E SEP241974 "CD 3 v S a mmam OUTER DIAM.

INNER DIAM.

REPLACEABLE ELECTRODE SURFACES FOR HIGH FIELD ELECTROSTATIC LENSES BACKGROUND OF THE INVENTION a. Field of Invention.

This invention relates generally to the field of highvoltage electrodes and, more particularly, to replaceable electrodes in an elctrostatic lens.

b. Discussion of Prior Art.

Focused electron and ion beams can be used in a large number of applications. Examples of such uses are for information recording, for scanning ion microscopes, and for writing out circuit patterns for integrated circuits.

The primary problem in using electron and ion beams is in focusing the beam to a small-enough cross-section, and deflecting the beam so as to convey information. When the beam is focused to a small-enough crosssection and is deflected in a programmed manner, the beam can be used in widely diverse applications.

It has been found that high-field-strength einsel lenses have advantages for the fine focusing of ion and electorn beams. This is especially so for ion beams where magnetic lenses are unfeasible and for electron beams in cases requiring rapid dynamic focusing in which magnetic lenses respond too slowly.

When electrostatic lenses, such as einsel lenses, are used to focus a beam to a small spot, they are used at a high field strength and a short focal length. In some instances, the full beam voltage is applied to the center electrode of the einsel lens. The two outer electrodes are at ground potential. It can be shown that for minimum spherical aberration, the lens size should be made as small as possible. The limiting dimension is thus the gap between the center (high voltage) electrode and the two outside (grounded) electrodes. This is due to the fact that only a limited amount of voltage can be applied across a small gap before arcing develops.

It is well known that high voltage breakdown in a vacuum is minimized if the electrodes are polished. Thus, generally, arcing will not occur between the electrodes until the highly polished electrodes become pitted or have deposits formed on them.

Pitting is caused by the fact that it is impossible to obtain a perfect vacuum. In the high field regions between the lens electrodes, electron collisions with individual gas molecules present in the vacuum system create residual ions. There ions are drawn to the electrodes and thus cause pitting of these electrodes.

Deposits can form on the elctrodes in a number of ways. For example, dust may be deposited on the electrode surfaces during exposure to air or hydrocarbons may polymerize on the positive electrode surfaces which usually experience electron bombardment.

In operation, after some initial run-in, a clean set of electrodes will operate arc-free for a limited time. The operation then deteriorates, either due to deposits or due to pitting of the electrodes. Electron currents on the order of IOOptA can be drawn by the positive center electrode at lOOkV when in a quiescent condition. During arcs, very large current surges occur.

In the past, the typical procedure to recondition the contaminated one piece electrodes was to remove them, polish them on a lathe, and re-install them in accurate alignment. The overall reconditioning proccess took about two days. A significant part of this time was used to re-align the electrodes. The electrode assembly has been redesigned in the present invention to reduce the time needed for lens reconditioning.

SUMMARY OF THE INVENTION Briefly, the electrode assembly of the present invention consists of mounting fixtures which are attached permanently to an electrostatic lens system, and replaceable polished electrode caps which are affixed to the mounting fixtures in such a way that they can be easily removed. Thus when the electrode caps becomes pitted, or have deposits developed on them, the electrode caps can be quickly and easily replaced.

OBJECTS OF THE INVENTION An object of the present invention is to substantially reduce the time needed for lens reconditioning in an electrostatic lens.

A further object of the present invention is to obviate the need to realign the electrodes in an electrostatic lens after each electrode reconditioning.

Yet another object is to design an electrode that can be quickly removed from its mounting in an electrostatic lens, and be easily replaced.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention whe considered in conjunction with the accompanying drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of the present invention in the basic einsel lens configuration.

FIGS. 2a and 2b show two views of a prior art, polished electrode surface.

FIG. 3 is a cross-sectional, side view of the electrode assembly of the present invention.

FIG. 4 is a cross-sectional side view of the mounting fixture 60.

FIG. 5 is a bottom view of the mounting fixture 60.

FIG. 6 is a cross-sectional side view of the mounting fixture 70.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I shows the basic einsel lens configuration. The tube 10 brings an electron or ion beam of approximately 15-20 mils in diameter into the region of the einsel lens system. It is desired to focus this beam to approximately a 1 micron diameter. In order to do this, the high-voltage electrode assembly 30 is given a very high potential, e.g., 100,000 volts, while outer electrode assemblies 20 and 40 are kept at ground potential. The forces due to this very high potential difference cause the beam to be focused.

The fixtures 60 are fastened to the tubes 10 by screws at points 55. Fixture is mounted directly to a highvoltage feed through. The replaceable electrode caps 50 contain the actual polished electrode surfaces that are subject to the high electric field. The ends of the caps 50 are made to conform to the fixtures 60 and 70 such that the caps 50 can be snapped into place easily. (In FIG. 1, the caps 50 and their respective fixtures 60 and 70 are separated by a small distance. This is done merely for the sake of clarity. Actually, these pieces fit very snugly together.)

The fixtures 60 and 70 are permanent. Thus once they are aligned and locked in place, no further realignment is needed.

When arcing occurs, the caps 50 can be easily pried off and either reconditioned or replaced with a new set of caps. The lens caps 50 are hydroformed to a high tolerance so that they are interchangeable. The time required to change a set of caps is about 1.5 hours, and most of this time is pump-down time. If the lens vacumm chamber is opened for another reason. these caps can be changed routinely before they fail, in a few minutes.

'FlGffshows two views or'apiib'raif'awraaiaed in an einsel lens, one view from below and one from above, This prior-art electrode is replaced by the replaceable electrode of FIG. 3 which is the subject of the present invention.

FIG. 3 shows a two-piece, electrode assembly consisting of a fixture piece 60, and a replaceable electrode cap 50.

FIGS. 4 and 5 show a side cross-sectional view and a top view of the mounting fixture 60. FIG. 5 gives a view of the side of the fixture (face 12 in FIG. 4) 60 to which the cap 50 will be affixed. A preferred embodiment has an inner diameter 1.350 inches, an outer diameter 4.000 inches. and a recess at some desired depth into fixture 60 with an ini i er diaineter 2.00 inches, and a outer diameter 3.00 inches. Further, there are four holes 66 drilled from the donut-shaped recess through to the other side of fixture 60. There are also four slots The purpose of the four slots, the donut-shaped recess, and the four holes is to allow any gases trapped between the cap 50 and the mounting fixtures 60 and 70 to escape since the pressure in this region is very important. It is very important to remove any trapped gases and thus improve the vacumm conditions. It has been found that the more metal-to-rnetal contact there is in a vacuum system, the more tapped gases there are. Thus the donut-shaped recess is used to reduce this metal-to-metal contact. The four slots and the four holes allow the trapped gases to escape.

Mounting fixture 70 has two caps 50 affixed to it. Thus mounting fixture 70, as shown in FIG. 6, can be thought of as two face 12s attached back-to-back. If, in FIG. 4, the bottom section of fixture 60 (dimension G) is attached to another bottom half (dimension G), then the fixture will be as in FIG. 6 with total thickness of 2 (G), or 0.5 inch in this case.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An electrode assembly to be used in an electrostatic lens system comprising:

mounting fixture means which is affixed permanently to said lens system;

a replaceable electrode cap affixed to said mounting fixture means,

said mounting fixture means and said replaceable electrode cap each being made in the shape of a donut, with the electron or ion beam on which said electrostatic lens system is acting being passed through the center of the donut-shaped mounting fixture means and electrode cap, and

said replaceable electrode cap being shaped to fit over one entire side of said mounting fixture means and to partially extend around the curved endcontour of said mounting fixture means so that said electrode cap snaps on and off said mounting fixture means in an easily removable manner.

2. An elctrode assembly as in claim 1 wherein said electrostatic lens system is an einsel lens system.

3. An electrode assembly as in claim 1 wherein said mounting fixture means has a recess cut to some de sired width and depth, extending completely around said mounting fixture means, said recess being formed on the side of said mounting fixture means to which said replaceable electrode cap is affixed, said recess having a plurality of holes extending from the bottom of said recess completely through said fixture means to its other side, said recess and said plurality of holes acting to allow any gases trapped between said replaceable lens cap and said mounting fixture means to escape.

4. An electrode assembly as in claim 3 wherein a plurality of slots are cut at some desired width and depth on the same side of said mounting fixture as said recess, and extending from the inner radius to the outer radius o s iqm um fiz rs,

5. An electrode assembly for use in an electrostatic lens system comprising:

mounting fixture means comprising a piece of material formed with a hole therethrough and having at least one side-surface through which said hole passes; and

replaceable, electrode cap means formed with a hole therethrough, said cap means being affixed to said mounting fixture means so that the holes in each are aligned for passage of an electron or ion beam therethrough, said electrode cap means being made to fit over said entire side-surface of said mounting fixture means through which said hole passes, and partially extending around the curved end-contour of said mounting fixture means so that said electrode cap snaps on and off said mounting fixture means in an easily removable manner.

6. An electrode assembly as in claim 5, wherein said mounting fixture means has a recess cut to some desired width and depth into the side-surface of said mounting fixture means which circumscribes said hole, said mounting fixture means also being formed with a plurality of slots cut at some desired width and depth extending from the inner radius to the outer radius of said mounting fixture means. whereby gases trapped between said cap and said surface may be vented. 7. An electrode assembly as in claim 5, said mounting fixture means having another surface opposed to the first through which said hole passes,

said assembly having another replaceable, electrode cap means like the first removably affixed to said other surface so that said hole passes through both cap means and said mounting fixture means.

8. An electrode assembly as in claim 6 wherein the opposing mounting fixture surface also contains a concentric recess and radial slots. 

1. An electrode assembly to be used in an electrostatic lens system comprising: mounting fixture means which is affixed permanently to said lens system; a replaceable electrode cap affixed to said mounting fixture means, said mounting fixture means and said replaceable electrode cap each being made in the shape of a donut, with the electron or ion beam on which said electrostatic lens system is acting being passed through the center of the donut-shaped mounting fixture means and electrode cap, and said replaceable electrode cap being shaped to fit over one entire side of said mounting fixture means and to partially extend around the curved end-contour of said mounting fixture means so that said electrode cap snaps on and off said mounting fixture means in an easily removable manner.
 2. An elctrode assembly as in claim 1 wherein said electrostatic lens system is an einsel lens system.
 3. An electrode assembly as in claim 1 wherein said mounting fixture means has a recess cut to some desired width and depth, extending completely around said mounting fixture means, said recess being formed on the side of said mounting fixture means to which said replaceable electrode cap is affixed, said recess having a plurality of holes extending from the bottom of said recess completely through said fixture means to its other side, said recess and said plurality of holes acting to allow any gases trapped between said replaceable lens cap and said mounting fixture means to escape.
 4. An electrode assembly as in claim 3 wherein a plurality of slots are cut at some desired width and depth on the same side of said mounting fixture as said recess, and extending from said recess to outer radius of said mounting fixture.
 5. An electrode assembly for use in an electrostatic lens system comprising: mounting fixture means comprising a piece of material formed with a hole therethrough and having at least one side-surface through which said hole passes; and replaceable, electrode cap means formed with a hole therethrough, said cap means being affixed to said mounting fixture means so that the holes in each are aligned for passage of an electron or ion beam therethrough, said electrode cap means being made to fit over said entire side-surface of said mounting fixture means through which said hole passes, and partially extending around the curved end-contour of said mounting fixture means so that said electrode cap snaps on and off said mounting fixture means in an easily removable manner.
 6. An electrode assembly as in claim 5, wherein said mounting fixture means has a recess cut to some desired width and depth into the side-surface of said mounting fixture means which circumscribes said hole, said mounting fixture means also being formed with a plurality of slots cut at some desired width and depth extending from said recess to the outer radius of said mounting fixture means, whereby gases trapped between said cap and said surface may be vented.
 7. An electrode assembly as in claim 5, said mounting fixture means having another surface opposed to the first through which said hole passes, said assembly having another replaceable, electrode cap means like the first removably affixed to said other surface so that said hole passes through both cap means and said mounting fixture means.
 8. An electrode assembly as in claim 6 wherein the opposing mounting fixture surface also contains a concentric recess and radial slots. 